This invention concerns a procedure and device for combining the transmission of information and radar function in a radar unit.
The main task of a radar unit is the detection of targets and the measurement of target data. In addition there might be a desire for a radar to be able to transmit information using its radar transmitter and antenna, that is to act as a data link terminal for the transmission of information.
Certain previous attempts to use a radar unit for the transmission of information in addition to its radar function were carried out mainly by using time multiplexing. Time multiplexing involves the radar unit at a certain moment in time working either as its main function as a radar unit or as a data link terminal, but not as both simultaneously.
The American patent U.S. Pat. No. 4,764,769 describes a system for the transmission of data using a radar unit that separates data pulses and radar pulses. The radar pulses are transmitted with a constant pulse repetition interval (PRI) with encoded information transmission in between. The encoded information transmission appears to use the radar pulses as reference timepoints. The aim of U.S. Pat. No. 4,764,769 appears to be the specific transmission of information that is required to remotely-control an unmanned vessel. The method does not appear to be suitable for any other application or for any other type of radar unit. The method impairs the performance of the radar function considerably and is not compatible with Doppler radar.
The British patent application GB 2129644 describes a system where the data encoding converts a binary one to a normal radar pulse and a binary zero to the omission of a radar pulse. This method affects the radar function and is not compatible with Doppler radar.
It can be regarded as a great disadvantage of all the above systems that the normal radar function is impaired when combined with data transmission or information transmission. The transmission of data makes the radar function either impossible or considerably poorer as the above systems work either as a radar system or as a transmission system. In order to improve this situation there have been developments towards systems where the radar unit transmits information at the same time as carrying out its main function as a radar unit.
In the European patent EP 289549 a method is given for the simultaneous transmission of information together with radar function. The method can be described briefly as follows: the transmitted signal is phase-modulated in one of two positions, 0.degree. or 180.degree., where the modulation is controlled by the information that is to be transmitted. When the information is received the phase position is evaluated to extract the transmitted information. Upon the reception of a target echo signal in the radar unit the phase position is reset by phase adjustment (information about the phase position is available from the previously transmitted signal). It can, however, be regarded as a disadvantage of this method that the transmitted signal is phase adjusted before transmission and then phase adjusted again as a target echo signal upon reception. Each signal processing (which is what a phase adjustment is) that the radar signal undergoes can distort the radar signal to a greater or lesser degree in addition to the intended change. This means that the target echo signal can show phase errors and other unwanted distortions due to the two phase adjustments. Certain types of radar such as low PRF pulse Doppler radar (LPD) are dependent upon a correct phase for best performance (PRF=pulse repetition frequency). The method in accordance with EP 289549 is not suitable for medium and high PRF pulse Doppler radar (MPD and HPD) at all, as a radar unit with any of these wave forms cannot distinguish target echo signals originating from different transmission pulses on account of the distance ambiguity. This problem can also arise using LPD with so-called second-time-round echoes. The method in accordance with EP 289549 therefore involves a deterioration of the performance of the radar unit.
A major problem with previous solutions for the transmission of information using a radar unit is thus being able to carry this out without at the same time impairing the performance of the radar function. Either the information transmission takes up valuable radar function time, that is to say the more information that is to be sent, the less time there is available for the radar function, or the information transmission impairs the performance of the radar function by distorting the radar signal in a way that is detrimental to the radar function.